Oled device having enhancement layer(s)

ABSTRACT

A method for improving the operation of an OLED includes maximizing on-radiative transfer of excited state energy from the OLED&#39;s organic emissive material to surface plasmon polaritons in an enhancement layer by providing the enhancement layer no more than a threshold distance away from the organic emissive layer; and emitting light into free space from the enhancement layer by scattering the energy from the surface plasmon polaritons through an outcoupling layer that is provided proximate to the enhancement layer but opposite from the organic emissive layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application claiming priorityunder 35 U.S.C. §119(e) of U.S. Provisional Application No. 62/092,909,filed Dec. 17, 2014, U.S. Provisional Application No. 62/078,585, filedNov. 12, 2014, and U.S. Provisional Application No. 62/028,509, filedJul. 24, 2014 the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a method of enhancing the operation ofan organic light emitting device (OLED).

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility and tolerance of disorder, may makethem well suited for particular applications such as fabrication on aflexible substrate. Examples of organic opto-electronic devices includeOLEDs, organic phototransistors, organic photovoltaic cells, and organicphotodetectors. For OLEDs, the organic materials may have performanceadvantages over conventional materials. For example, the wavelength atwhich an organic emissive layer emits light may generally be readilytuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the following structure:

In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled inthe art, a first work function is “greater than” or “higher than” asecond work function if the first work function has a higher absolutevalue. Because work functions are generally measured as negative numbersrelative to vacuum level, this means that a “higher” work function ismore negative. On a conventional energy level diagram, with the vacuumlevel at the top, a “higher” work function is illustrated as furtheraway from the vacuum level in the downward direction. Thus, thedefinitions of HOMO and LUMO energy levels follow a different conventionthan work functions.

As used herein, and as would be generally understood by one skilled inthe OLED art, the terms “emitter,” “emissive material,” “light emittingmaterial,” have the equivalent meaning and are used interchangeably.These materials are understood to encompass all organic materials thatare phosphorescent material, fluourescent material, thermally activateddelayed fluorescent material, chemi-luminescent material, and organicmaterials that exhibit all other classes of organic emission.

The term “sharp edges” as used herein refers to an edge formed betweentwo surfaces whose cross-section has a radius of curvature between 0 to10 nm, preferably 0 to 5 nm, and more preferably 0 to 2 nm.

The term “organic emissive layer” of an OLED as used herein refers tothe layer in an OLED comprised of a light emitting material or a lightemitting material and one or more hosts and/or other materials. Typicalorganic emissive layer thicknesses are from 0.5 to 100 nm, morepreferably 0.5 to 60 nm. When the organic emissive layer is composed ofa light emitting material and one or more hosts or other materials, thelight emitting material is doped into the emissive layer from 0.01 to40% by weight, more preferentially, 0.1 to 30% by weight, mostpreferably 1% to 20% by weight.

The term “wavelength-sized features” as used herein refers to featureswhose dimensions coincide with one or more of the intrinsic emissionwavelengths of the organic emissive material in the organic emissivelayer of an OLED. The term “sub-wavelength-sized” as used herein refersto features whose dimensions are smaller than any of the intrinsicemission wavelengths of the organic emissive material in the organicemissive layer of an OLED. Intrinsic emission wavelengths refers to thewavelengths the organic emissive material would emit if it were emittingin free space divided by the refractive index of the organic emissivelayer in the OLED.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

The use of surface plasmon polaritons or localized surface plasmonpolaritons for optoelectronic devices recently has been recognized inthe OLED industry. However, these systems rely on balancing thetrade-off between enhancing the radiative rate of the emitter andpreventing non-radiative energy transfer to the surface plasmon mode,also known as quenching. Both the radiative rate enhancement and thenon-radiative quenching are a strong function of the distance betweenthe light emitting material and the plasmonic material. To achieve aradiative rate enhancement previous reports utilize a dielectric spacinglayer between the light emitting material and the plasmonic materiallayer in order to prevent quenching. The exact thickness of thedielectric spacer layer depends on many factors including: thecomposition of the plasmonic material; the thickness of the plasmonicmaterial layer; whether the plasmonic material layer is patterned; thesurface roughness of the plasmonic material layer; in the case of theplasmonic material being provided in the form of nanoparticles, the sizeand shape of the nanoparticles; the dielectric constant of thedielectric spacer layer in contact with the plasmonic material layer;and the wavelength of the emission for the light emitting material.

SUMMARY OF THE INVENTION

According to an embodiment, a method for improving the operation of anOLED is disclosed where the OLED comprises an organic emissive layercomprising an organic emissive material. The method comprises maximizingnon-radiative transfer of excited state energy from the organic emissivematerial to surface plasmon polariton in an enhancement layer byproviding the enhancement layer, comprising a plasmonic materialexhibiting surface plasmon resonance that non-radiatively couples to theorganic emissive material, no more than a threshold distance away fromthe organic emissive layer, wherein the organic emissive material has atotal nonradiative decay rate constant and a total radiative decay rateconstant due to the presence of the enhancement layer and the thresholddistance is where the total non-radiative decay rate constant is equalto the total radiative decay rate constant; and emitting light into freespace from the enhancement layer by scattering the energy from thesurface plasmon polariton through an outcoupling layer that is providedproximate to the enhancement layer but opposite from the organicemissive layer. In another embodiment, an intervening layer is providedbetween the enhancement layer and the outcoupling layer.

According to another embodiment, an enhanced OLED is disclosed. The OLEDcomprises: a substrate; a first electrode; an organic emissive layercomprising an organic emissive material disposed over the electrode; anenhancement layer, comprising a plasmonic material exhibiting surfaceplasmon resonance that non-radiatively couples to the organic emissivematerial and transfers excited state energy from the emissive materialto non-radiative mode of surface plasmon polariton, disposed over theorganic emissive layer opposite from the first electrode, wherein theenhancement layer is provided no more than a threshold distance awayfrom the organic emissive layer, wherein the organic emissive materialhas a total nonradiative decay rate constant and a total radiative decayrate constant due to the presence of the enhancement layer and thethreshold distance is where the total non-radiative decay rate constantis equal to the total radiative decay rate constant; and an outcouplinglayer disposed over the enhancement layer, wherein the outcoupling layerscatters the energy from the surface plasmon polaritons as photons tofree space. In other embodiments, an intervening layer is disposedbetween the enhancement layer and the outcoupling layer, wherein theintervening layer has a thickness between 1-10 nm and a refractive indexfrom 0.1 to 4.0.

According to another embodiment, an enhanced OLED comprises: asubstrate, wherein the substrate can be transparent; a first outcouplinglayer disposed over the substrate; a first enhancement layer disposed onthe first outcoupling layer; an organic emissive layer comprising anorganic emissive material disposed over the first enhancement layer,wherein the first enhancement layer comprising a first plasmonicmaterial exhibiting surface plasmon resonance that non-radiativelycouple to the organic emissive material and transfer excited stateenergy from the organic emissive material to non-radiative mode ofsurface plasmon polaritons, wherein the first enhancement layer isprovided no more than a threshold distance away from the organicemissive layer; a second enhancement layer disposed over the organicemissive layer, the second enhancement layer comprising a secondplasmonic material exhibiting surface plasmon resonance thatnon-radiatively couple to the organic emissive material and transferexcited state energy from the organic emissive material to non-radiativemode of surface plasmon polaritons, wherein the second enhancement layeris provided no more than the threshold distance away from the organicemissive layer, wherein the organic emissive material has a totalnon-radiative decay rate constant and a total radiative decay rateconstant due to the presence of the first and second enhancement layersand the threshold distance is where the total non-radiative decay rateconstant is equal to the total radiative decay rate constant; the secondenhancement layer comprising a second plasmonic material exhibitingsurface plasmon resonance that nonradiatively couple to the organicemissive material and transfer excited state energy from the emissivematerial to non-radiative mode of surface plasmon polaritons; and asecond outcoupling layer disposed over the second enhancement layer,wherein the first and second outcoupling layers scatter the energy fromthe surface plasmon polaritons as photons to free space. In otherembodiments, the enhanced OLED further comprises a first interveninglayer disposed over the first outcoupling layer between the firstoutcoupling layer and the first enhancement layer; and a secondintervening layer disposed over the second enhancement layer between thesecond enhancement layer and the second outcoupling layer.

The enhancement layer modifies the effective properties of the medium inwhich the organic fluorophore or phosphorescent molecule residesresulting in any or all of the following: a decreased rate of emission,a modification of emission line-shape, a change in emission intensitywith angle, a change in aging rate of emitter, and reduced efficiencyroll-off of the OLED device. Placement of the enhancement layer at thecathode side, anode side, or on both sides results in OLED devices whichtake advantage of any of the above mentioned effects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows a qualitative plot of the radiative decay rate constant dueto the surface plasmon polariton (SPP) mode and the non-radiative decayrate constant due to the SPP mode as a function of distance of the lightemitting material from the metallic film. Also plotted is the ratio ofthe total radiative to non-radiative rate constant for the system as afunction of distance of the light emitting material from the metallicfilm which is dominated by the rate constants due to the SPP mode.

FIG. 4A shows a plot of quantum yield as a function of light emittingmaterial's distance from the metallic enhancement film with twothreshold distances identified.

FIG. 4B shows a schematic depiction of the temperature of the OLED as afunction of the light emitter's distance from the metallic enhancementfilm when there is no outcoupling layer for the non-radiative OLED withthe threshold distance 2 identified on the plot.

FIG. 5 is a schematic illustration of an example of enhancement layercomprised of three unit cells.

FIG. 6 is a schematic illustration of an example design for the unitcell which comprises the enhancement layer. Each subcomponent layer ofthe unit cell may be composed of different materials, as shown.

FIG. 7 is a schematic illustration of an example enhancement layerdesign.

FIGS. 8A-8C show schematic illustrations of examples of OLED deviceswith enhancement layer(s). The OLED devices feature individual anode andcathode contact layers. The OLED can be top or bottom emitting.

FIG. 9A-9E show schematic illustrations of examples of OLED devices withenhancement layer(s). The enhancement layer(s) act as contact(s) for theOLED devices. The OLED can be top or bottom emitting.

FIG. 10 is a schematic illustration of an example of an enhancementlayer OLED device structure according to one embodiment.

FIG. 11 is a schematic illustration of an example of an enhancementlayer OLED device structure according to another embodiment.

FIG. 12 is a plot showing the enhancement factor of the light emitter'srate constant as a function of emitter's peak wavelength and the totalOLED thickness.

FIG. 13A is a top down view of a 2D patterned enhancement layer.

FIG. 13B is a top down view of a 3D patterned enhancement layer.

FIG. 14A shows optical modeling data for emission rate constant versuswavelength for the preferred structure shown in FIG. 7 demonstrating thepredicted broadband increase in emission rate constant.

FIG. 14B shows optical modeling data for emission intensity versuswavelength for the green emitter within the structure demonstratingnarrow emission due to the Purcell effect.

FIG. 14C shows optical modeling data for outcoupled fraction of emissionversus wavelength for various hole transport layer thicknesses.

FIG. 15 shows an emission from experimentally realized preferredstructure shown in FIG. 7 for various hole transport layer thicknesses.

Other than the plots shown in FIGS. 3, 4A, 4B, 12, 14A-C, and 15 allfigures are illustrated schematically and are not intended to showactual dimensions or proportions. In addition to the specific functionallayers mentioned herein and illustrated in the various OLED examplesshown in the figures, the OLEDs according to the present disclosure mayinclude any of the other functional layers often found in OLEDs.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence is described in moredetail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporatedby reference.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, a cathode 160, and a barrier layer 170.Cathode 160 is a compound cathode having a first conductive layer 162and a second conductive layer 164. Device 100 may be fabricated bydepositing the layers described, in order. The properties and functionsof these various layers, as well as example materials, are described inmore detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which areincorporated by reference.

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is m-MTDATA dopedwith F₄-TCNQ at a molar ratio of 50:1, as disclosed in U.S. PatentApplication Publication No. 2003/0230980, which is incorporated byreference in its entirety. Examples of emissive and host materials aredisclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which isincorporated by reference in its entirety. An example of an n-dopedelectron transport layer is BPhen doped with Li at a molar ratio of 1:1,as disclosed in U.S. Patent Application Publication No. 2003/0230980,which is incorporated by reference in its entirety. U.S. Pat. Nos.5,703,436 and 5,707,745, which are incorporated by reference in theirentireties, disclose examples of cathodes including compound cathodeshaving a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thetheory and use of blocking layers is described in more detail in U.S.Pat. No. 6,097,147 and U.S. Patent Application Publication No.2003/0230980, which are incorporated by reference in their entireties.Examples of injection layers are provided in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety. A description of protective layers may be found in U.S. PatentApplication Publication No. 2004/0174116, which is incorporated byreference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of nonlimiting example, and it is understood that embodiments of theinvention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove outcoupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. Pat. No. 7,431,968, which is incorporated by reference in itsentirety. Other suitable deposition methods include spin coating andother solution based processes. Solution based processes are preferablycarried out in nitrogen or an inert atmosphere. For the other layers,preferred methods include thermal evaporation. Preferred patterningmethods include deposition through a mask, cold welding such asdescribed in U.S. Pat. Nos. 6,294,398 and 6,468,819, which areincorporated by reference in their entireties, and patterning associatedwith some of the deposition methods such as ink jet and OVJD. Othermethods may also be used. The materials to be deposited may be modifiedto make them compatible with a particular deposition method. Forexample, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the presentinvention may further optionally comprise a barrier layer. One purposeof the barrier layer is to protect the electrodes and organic layersfrom damaging exposure to harmful species in the environment includingmoisture, vapor and/or gases, etc. The barrier layer may be depositedover, under or next to a substrate, an electrode, or over any otherparts of a device including an edge. The barrier layer may comprise asingle layer, or multiple layers. The barrier layer may be formed byvarious known chemical vapor deposition techniques and may includecompositions having a single phase as well as compositions havingmultiple phases. Any suitable material or combination of materials maybe used for the barrier layer. The barrier layer may incorporate aninorganic or an organic compound or both. The preferred barrier layercomprises a mixture of a polymeric material and a non-polymeric materialas described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporatedby reference in their entireties. To be considered a “mixture”, theaforesaid polymeric and non-polymeric materials comprising the barrierlayer should be deposited under the same reaction conditions and/or atthe same time. The weight ratio of polymeric to nonpolymeric materialmay be in the range of 95:5 to 5:95. The polymeric material and thenonpolymeric material may be created from the same precursor material.In one example, the mixture of a polymeric material and a non-polymericmaterial consists essentially of polymeric silicon and inorganicsilicon.

Devices fabricated in accordance with embodiments of the invention canbe incorporated into a wide variety of electronic component modules (orunits) that can be incorporated into a variety of electronic products orintermediate components. Examples of such electronic products orintermediate components include display screens, lighting devices suchas discrete light source devices or lighting panels, etc. that can beutilized by the end-user product manufacturers. Such electroniccomponent modules can optionally include the driving electronics and/orpower source(s). Devices fabricated in accordance with embodiments ofthe invention can be incorporated into a wide variety of consumerproducts that have one or more of the electronic component modules (orunits) incorporated therein. Such consumer products would include anykind of products that include one or more light source(s) and/or one ormore of some type of visual displays. Some examples of such consumerproducts include flat panel displays, computer monitors, medicalmonitors, televisions, billboards, lights for interior or exteriorillumination and/or signaling, heads-up displays, fully or partiallytransparent displays, flexible displays, laser printers, telephones,cell phones, tablets, phablets, personal digital assistants (PDAs),laptop computers, digital cameras, camcorders, viewfinders,micro-displays, 3-D displays, vehicles, a large area wall, theater orstadium screen, or a sign. Various control mechanisms may be used tocontrol devices fabricated in accordance with the present invention,including passive matrix and active matrix. Many of the devices areintended for use in a temperature range comfortable to humans, such as18 degrees C. to 30 degrees C., and more preferably at room temperature(20-25 degrees C.), but could be used outside this temperature range,for example, from −40 degree C. to +80 degree C.

According to an aspect of the present disclosure, unlike theconventional teachings that sought to prevent or inhibit exciton energytransfer to the non-radiative mode of the surface plasmon polaritons(“SPP”) in the metal electrodes, as that energy is typically lost, thedisclosed method intentionally puts as much energy as possible into thenon-radiative mode and then extracts that energy to free space as lightusing an outcoupling layer.

According to an aspect of the present disclosure, a method for improvingthe operation of an OLED wherein the OLED comprises an organic emissivelayer comprising an organic emissive material is disclosed. The methodcomprises maximizing non-radiative transfer of excited state energy fromthe organic emissive material to surface plasmon polaritons in anenhancement layer by positioning the enhancement layer, comprising aplasmonic material exhibiting surface plasmon resonance thatnon-radiatively couples to the organic emissive material, no more than athreshold distance away from the organic emissive layer, wherein theorganic emissive material has a total non-radiative decay rate constantand a total radiative decay rate constant due to the presence of theenhancement layer and the threshold distance is where the totalnon-radiative decay rate constant is equal to the total radiative decayrate constant; and emitting light into free space from the enhancementlayer by scattering the energy from the surface plasmon polaritonsthrough an outcoupling layer provided adjacent to the enhancement layer.It is expected that the closer the emissive material is to theenhancement layer the greater the OLED performance will be. In someembodiments, an intervening layer is provided between the enhancementlayer and the outcoupling layer for tuning the wavelength of light thatis outcoupled to free space by the outcoupling layer.

After the exciton energy from the emitter is fully captured in thenon-radiative mode of the SPP, the energy is emitted as light into freespace from the enhancement layer by scattering the energy from the SPPthrough the outcoupling layer.

The enhancement layer can be comprised of plasmonic materials, opticallyactive metamaterials, or hyperbolic metamaterials. As used herein, aplasmonic material is a material in which the real part of thedielectric constant crosses zero in the visible or ultraviolet region ofthe electromagnetic spectrum. In general, a metamaterial is a mediumcomposed of different materials where the medium as a whole actsdifferently than the sum of its material parts. In particular, we defineoptically active metamaterials as materials which have both negativepermittivity and negative permeability.

Hyperbolic metamaterials, on the other hand, are anisotropic media inwhich the permittivity or permeability are of different sign fordifferent spatial directions. Optically active metamaterials andhyperbolic metamaterials are strictly distinguished from many otherphotonic structures such as Distributed Bragg Reflectors (“DBRs”) inthat the medium should appear uniform in the direction of propagation onthe length scale of the wavelength of light. Using terminology that oneskilled in the art can understand: the dielectric constant of themetamaterials in the direction of propagation can be described with theeffective medium approximation. Plasmonic materials and metamaterialsprovide methods for controlling the propagation of light that canenhance OLED performance in a number of ways.

In some embodiments, the enhancement layer is provided as a film layerof the materials mentioned above. In one preferred embodiment, theenhancement layer is provided as at least one set of gratings formed ofwavelength-sized features that are arranged periodically,quasi-periodically, or randomly, or sub-wavelength-sized features thatare arranged periodically, quasi-periodically, or randomly. In anotherpreferred embodiment, the wavelength-sized features and thesub-wavelength-sized features have sharp edges.

A grating refers to any regularly spaced collection of essentiallyidentical, parallel, elongated elements. Gratings usually consist of asingle set of elongated elements, but can consist of two sets, in whichcase the second set is usually oriented at a different angle relative tothe first set. For example, the second set can be oriented orthogonal tothe first set. The grating embodiment for the enhancement layer will bedescribed in further detail below.

The increased emission rate constant of the OLED emitter is stronglydependent on the distance of the emitter from the enhancement layer.Once the emitter is closer than a threshold distance, achieving betterperformance requires moving the light emitting material closer to theenhancement layer. To achieve a better OLED performance, the preferreddistance from the enhancement layer to the organic emissive layercontaining the emissive material (“EML”) is not greater than 100 nm,more preferably not greater than 60 nm, and most preferably not greaterthan 25 nm. According to another aspect of the present disclosure, insome cases, for manufacturability reasons it may be desirable to havethe distance between the enhancement layer and the EML to be 5-100 nm,more preferably 5-60 nm, and most preferably 5-25 nm. Achieving thisdesired distance between the EML and the enhancement layer may requireproviding one or more of the various functional OLED layers between theEML and the enhancement layer so that the EML and the enhancement layerare not in direct contact. Such functional OLED layers are well known inthe art. For example, one may include a hole injection layer between theenhancement layer and the emissive material layer to lower the voltageof operation of the OLED. FIGS. 10 and 11 show OLED device architecturesshowing the various functional OLED layers that may be optionallyprovided between the EML and the enhancement layer. The minimumthreshold distance in this embodiment is desired to be 5 nm becauseinventors have found that that is about the minimum thickness requiredfor the various materials to form a working functional OLED layer.

Understanding the advantages of using the non-radiative mode of SPP andcontrolling the distance between the EML and the enhancement layer to benot greater than the threshold distance begins with the decay rateconstants of the light emitting material. For any light emitter theQuantum Yield (QY) of photons can be expressed as the ratio of theradiative and nonradiative decay rate constants and is explicitlydefined as the number of photons emitted per excited state:

$\begin{matrix}{{QY} = \frac{k_{rad}^{total}}{k_{rad}^{total} + k_{{non}\text{-}{rad}}^{total}}} & (1)\end{matrix}$

where k_(rad) ^(total) is the sum of all the radiative processes andk_(non-rad) ^(total) is the sum of all the nonradiative processes. Foran isolated emitter in free space, we can define the molecular radiativeand non-radiative decay rate constants, k⁰ _(rad) and k⁰ _(non-rad). Forthe isolated molecule, the Quantum Yield (QY°) is:

$\begin{matrix}{{QY}^{0} = \frac{k_{rad}^{0}}{k_{rad}^{0} + k_{{non}\text{-}{rad}}^{0}}} & (2)\end{matrix}$

However, in an optoelectronic device, such as an OLED, there are anumber of other processes which affect the total radiative andnon-radiative decay rate constants. Some of these are energy transfer tothe radiative and non-radiative decay modes of the surface plasmon in aplasmonic material such as a metal. These modes become important whenthe light emitting material is in the vicinity of the plasmonicmaterial. This leads to increased values for both the total radiativedecay rate constant and total non-radiative decay rate constant in thepresence of a plasmonic material. In the quantum yield, these additionalprocesses can be specifically accounted for:

$\begin{matrix}{{QY} = {\frac{k_{rad}^{total}}{k_{rad}^{total} + k_{{non}\text{-}{rad}}^{total}} = \frac{k_{rad}^{0} + k_{rad}^{plasmon}}{k_{rad}^{0} + k_{rad}^{plasmon} + k_{{non}\text{-}{rad}}^{0} + k_{{non}\text{-}{rad}}^{plasmon}}}} & (3)\end{matrix}$

where k_(rad) ^(plasmon) and k_(non-rad) ^(plasmon) are the radiativeand non-radiative decay rate constants, respectively, for the lightemitter when interacting with the SPP.

A qualitative plot of k_(rad) ^(plasmon) and k_(non-rad) ^(plasmon) as afunction of the light emitter's distance from an enhancement layer suchas a metallic film is shown in FIG. 3. For distances close to theenhancement layer, k_(rad) ^(total) will be dominated by the k_(rad)^(plasmon) term and k_(non-rad) ^(total) will be dominated byk_(non-rad) ^(plasmon). Thus, FIG. 3 also describes the total radiativeand non-radiative decay rates. This is shown on the right axis of FIG. 3which is the ratio of the total radiative decay rate constant, k_(rad)^(total), to the total non-radiative decay rate constant, k_(non-rad)^(total). As is shown on the left axis of FIG. 3, the two plasmon baseddecay rate constants have different functional dependencies on thedistance of the emitter from the metallic layer (in this case 1/r̂6 forthe nonradiative and 1/r̂3 for the radiative rate). The differentdependencies on distance from the enhancement layer results in a rangeof distances over which the radiative decay rate constant is the largestrate constant due to the interactions with the surface plasmon. Fordistances within this particular range, the photon yield is increasedover the photon yield of an isolated molecule without the enhancementlayer, increasing the QY. This is illustrated in the plot FIG. 4A inwhich quantum yield is plotted as a function of light emittingmaterial's distance from the metallic film. Once the non-radiative decayrate constant becomes near in value to the radiative decay rate the QYstarts to drop, creating a peak in the QY at some specific distance.

For a given pair of light emitting material and enhancement layer, thereis a total nonradiative decay rate constant and a total radiative decayrate constant. As the light emitting material layer becomes closer tothe enhancement layer, the non-radiative decay rate constant grows morerapidly than the radiative decay rate constant. At some distance, thetotal nonradiative decay rate constant of the light emitting material inthe presence of the enhancement layer is equal to the total radiativedecay rate constant of the light emitting material in the presence ofthe enhancement layer. This will be referred to herein as the ThresholdDistance 1. Threshold Distance 1 is the distance the light emittinglayer is from the enhancement layer at which the following statementholds:

k _(non-rad) ^(plasmon) +k _(non-rad) ⁰ =k _(rad) ^(plasmon) +k _(rad)⁰  (4)

For distances closer to the enhancement layer than the ThresholdDistance 1 the total non-radiative decay rate is larger than theradiative decay rate and the quantum yield is less than 0.5 or 50%. Forthese distances, there is an even larger speed-up in the rate at whichenergy leaves the light emitter as the non-radiative decay rate constantexceeds the radiative decay rate constant. The enhancementlayer-to-emitter distances less than or equal to the Threshold Distance1 satisfy the following condition:

k _(non-rad) ^(plasmon) ≧k _(rad) ^(plasmon) +k _(rad) ⁰ −k _(non-rad)⁰  (5)

For distances larger than Threshold Distance 1 the total radiative decayrate constant is larger than the total non-radiative decay rateconstant, however, the quantum yield of the light emitting material isreduced over the case when the enhancement layer is not present. Thusthe light emitter is still quenched, a process which is avoided intypical opto-electronic devices but is essential to the operation ofthis invention.

The distance of the emitter from the enhancement layer at whichquenching starts is defined herein as the Threshold Distance 2. At thisdistance, the QY of the light emitter in the presence and absences ofthe enhancement layer is identical. When the light emitter is movedcloser to the enhancement layer the QY drops. Threshold Distance 2 isthe distance the light emitting layer is from the enhancement layer atwhich the following statement holds:

QY⁰=QY  (6),

where QY⁰ is the light emitting materials intrinsic quantum yield and QYis the quantum yield with the enhancement layer. This leads to thefollowing expression when accounting for the radiative and non-radiativedecay rate constants due to the plasmonic material of the enhancementlayer:

$\begin{matrix}{\frac{k_{rad}^{0}}{k_{rad}^{0} + k_{{non}\text{-}{rad}}^{0}} = \frac{k_{rad}^{0} + k_{rad}^{plasmon}}{k_{rad}^{0} + k_{{non}\text{-}{rad}}^{0} + k_{rad}^{plasmon} + k_{{non}\text{-}{rad}}^{plasmon}}} & (7)\end{matrix}$

Solving the k_(non-rad) ^(plasmon) we obtain the following expressionfor the decay rate constants at Threshold Distance 2:

$\begin{matrix}{k_{{non}\text{-}{rad}}^{plasmon} = {\frac{k_{{non}\text{-}{rad}}^{0}}{k_{rad}^{0}} \cdot k_{rad}^{plasmon}}} & (8)\end{matrix}$

Conceptually Threshold Distance 2 is more easily understood from thevalue of the quantum yield being equal to the value of the quantum yieldof the light emitting material without the enhancement layer. See FIG.4A.

Whether the Threshold Distance 1 or Threshold Distance 2 are considered,the physical values of the threshold distances depend on a number offactors including the frequency of the surface plasmon polariton,oscillator strength of the light emitting material, and the dielectricconstant of the light emitting material layer. Therefore, by selecting asuitable set of materials for the organic light emitting material andthe plasmonic material of the enhancement layer, the threshold distancecan be adjusted.

Measurable Parameters for a Non-Radiative Energy Transfer OLED

The non-radiative energy transfer OLED can be distinguished from otherplasmonic OLEDs by a measurement of the quantum yield of the lightemitting material. One would fabricate a number of OLEDs or devices withthe enhancement layer but not electrodes with varying enhancementlayer-to-emitter distances and one sample with no enhancement layer atall. For each sample, the quantum yield which is defined as the numberof photons emitted per each photon absorbed is measured. Thresholddistance 1 is the distance at which the total radiative decay rateconstant is equal to the total non-radiative decay rate constant, atthis distance the photoluminescent quantum yield will be 50% without theenhancement layer. At threshold distance 2 the quantum yield will be thesame as the value without the enhancement layer, see FIG. 4A. Anadditional measure of whether the SPP coupling is increasing theradiative or the non-radiative rate is to measure the temperature of theOLED. Since non-radiative quenching of the exciton generates heatinstead of photons, the OLED will heat up. The heat generated in theOLED will be proportional to the yield of non-radiatively recombinedexcitons when there is no outcoupling layer:

$\begin{matrix}{{{Heat}\mspace{14mu} {yield}} \propto {\frac{k_{{non}\text{-}{rad}}^{total}}{k_{rad}^{total} + k_{{non}\text{-}{rad}}^{total}}.}} & (9)\end{matrix}$

As the distance between the light emitter and the metallic film isvaried, the total heat conduction of the OLED will remain essentiallyconstant, however, the heat yield will vary greatly.

FIG. 4B schematically illustrates the steady state temperature of theOLED as the distance between the light emitter and metallic film layeris varied for a fixed current density of operation. For large distancesof the emitting layer from the metallic surface, there is no enhancementof the radiative or non-radiative decay rate constants. The temperatureof the OLED depends only on the total current density of operation andthe efficiency of the light emitting material. As the emitter is broughtcloser to the metallic film layer, the radiative decay rate constantincreases and the photon yield increases, reducing the heat generated inthe OLED and the OLED's steady state temperature. For distances shorterthan the threshold distance 2, the excitons on the light emitter arequenched as heat and the OLED's normalized temperature increases.

There may be others but the inventors suggest here two temperaturerelated experiments as examples that can be performed to determinewhether the light emitter is positioned no greater than the thresholddistance 2 from the enhancement layer such that the nonradiative surfaceplasmon rate constant is large enough to induce quenching. When thenonradiative decay rate constant is inducing quenching the OLED willheat up more than when the radiative decay rate constant is increasingthe quantum yield. In the first experiment, the operating temperature ofthe OLEDs is measured for a number of OLEDs with varying metallicfilm-to-emitter distances. For each device operating at a fixed currentdensity, the temperature of the OLED is measured. The OLED will heat upmore as the metallic film-to-emitter distance gets shorter thanthreshold distance 2. In the second experiment, one can build a controlOLED in which the metallic film is replaced with a transparentconducting oxide which does not have a strong surface plasmon resonance.One such material is ITO. One can measure and compare the temperature ofthe control device having the ITO layer against another device havingthe metallic film. If the temperature of the OLED with the metallic filmis higher than the control device with ITO, then, the non-radiative rateis dominant and the distance between the metallic film and the emitteris not greater than the threshold distance 2.

To provide the enhancement layer at a distance not greater than eitherthreshold distance from the organic emissive layer, one of a number ofother OLED functional layers can be provided in the space between theenhancement layer and the organic layer. Such OLED functional layers arewell know to those skilled in the art. Some examples of such OLEDfunctional layers are illustrated in FIGS. 10 and 11, for example.

Increases in OLED performance occur due to the excited state of thelight emitting material energy transferring to the enhancement layer.Enhanced dipole moment coupling of the OLED fluorophore orphosphorescent molecule to the enhancement layer results in (1) anincreased emission rate constant for the emitter and (2) increasedenergy transferred from the molecule to the enhancement layer.

The phenomenon (1) is a result of the unique optical properties of theenhancement layer. The enhancement layer modifies the modes that theemitter experiences increasing the density of states over some spectralrange. The increased density of states increases the emitter's radiativeand non-radiative decay rate constants. An increased density of stateschanges the emission spectrum of the emitter if the increase in densityof states is not broadband. These effects are referred to as the Purcelleffect.

Hyperbolic metamaterials and plasmonic materials are well suited toenhancing the Purcell effect in an OLED as they have broadband photonicstates. This is in contrast to DBRs or microcavities where the increasein photon density of states occurs over a narrow spectral window.Hyperbolic metamaterials have been used to increase the emissive rateconstant and the emission intensity of quantum dots and laser dyes.

Although hyperbolic metamaterials and plasmonic materials areintrinsically broadband, they can be made narrow, or resonant, throughpatterning. Patterning can occur either laterally or vertically.Patterning is discussed in more detail below.

Thus, phenomenon (1) is a result of the increase in photonic states thatthe emitter can sample by introducing the enhancement layer into theOLED. Phenomenon (1) then results in changes to the OLEDs performance.An increased radiative decay rate constant results in a lower excitondensity within the emissive layer of the OLED device for a given currentdensity relative to a device without the enhancement layer. This reducesloss mechanisms which rely on two particle collisions such as tripletannihilation and triplet-charge annihilation at high brightness, thusimproving OLED performance at high brightness. An increase in theemission rate constant of the emitter will also reduce the average timethe emitter spends in the excited state, reducing the total energystored in the OLED for a given current density of operation. It isexpected this will leads to a reduce rate at which the molecule agesinto a non-luminescent species, resulting in a longer lifetime for theOLED device.

There are substantial differences between the Purcell effect due to theenhancement layer and an optical microcavity. The first is that thelayer thicknesses (physical or optical) are very different.

The enhancement layer effect does not depend on the total thickness ofthe cavity, only the distance from the emitter to the enhancement layer.In addition, there are no nodes of increased performance as occurs inmicrocavities which depend on constructive and interference of theemitted photon with the partially reflective mirrors. Instead, theincrease in performance is reduced as the light emitting material ismoved farther from the enhancement layer. The enhancement layer does notneed to be a mirror or mirror-like or even partially reflective.Finally, the thickness of the enhancement layer does not need to be inthe order of the wavelength of light to modify the properties of theOLED emitter as is the case for a distributed bragg reflector.

Phenomenon (2) results in greater total energy dissipation of theemitting molecule to the enhancement layer. This reduces emission lostto the substrate mode and contact layers. Assuming that light is coupledfrom the enhancement layer to free space, phenomenon (2) results in agreater number of photons emitted per electron. These two results workindependently or in tandem to increase OLED performance.

Preferably, the enhancement layer should appear uniform to thewavelength of light, in contrast to typical DBR and microcavity devicespreviously used in OLEDs. As mentioned above, the enhancement layer canbe formed of one or more of metallic film, an optically activemetamaterial, and a hyperbolic metamaterial, or any combinations ofthem, for example.

As mentioned above, in some embodiments, the enhancement layer can be asingle layer of metallic film, an optically active metamaterial, and ahyperbolic metamaterial.

Additional increases in performance may be achieved when the enhancementlayer is patterned. Preferably, such patterned enhancement layer is atleast one set of gratings formed of wavelength-sized features that arearranged periodically, quasi-periodically, or randomly, orsub-wavelength-sized features that are arranged periodically,quasi-periodically, or randomly. In one preferred embodiment, thewavelength-sized features and the sub-wavelength-sized features can havesharp edges.

Patterning of the enhancement layer may increase device performance inmultiple ways. First, patterning the enhancement layer withwavelength-sized or sub-wavelength-sized features having sharp edgeswill produce fringing fields that will couple more efficiently tohorizontally oriented molecular dipoles of the emissive species. Thiswill increase the nonradiative decay rate constant of the light emittingmaterial, potentially increasing the durability of the emitter and thetotal amount of light outcoupled to air.

The second performance enhancement arises when patterning theenhancement layer with periodic wavelength-sized features which createsresonant plasmonic modes. As the periodicity of the patterning increasesthe bandwidth of the surface plasmon mode can be reduced. Highlyperiodic structures such as plasmonic materials patterned with aperiodic array of holes creates resonant plasmonic modes which may haveincreased quality factors, narrower spectral width, or lower loss. Thereduced bandwidth of the surface plasmon mode allows for selectivecoupling of the surface plasmon to particular wavelengths of emissionwhile potentially leaving other wavelength emitters un-altered. Theselective coupling may be useful for device structures with multipleemitters such as white OLEDs and stack OLEDs. Use of random orquasi-periodic wavelength-sized patterns on the enhancement layer couldbe used for increasing the bandwidth of response in the context ofmultiwavelength emitters or broadband emitters.

Finally, patterning of the enhancement layer can facilitate theoutcoupling of the nonradiative mode of the surface plasmon to air. Thiscan be achieved by either directly scattering energy from thenon-radiative mode of the surface plasmon to air due to the patterningor through increasing the coupling between the non-radiative mode of thesurface plasmon and the outcoupling layer. In some embodiments, thepatterning of the enhancement layer can be configured to outcoupleenergy from the non-radiative mode of the surface plasmon to air for allwavelengths in the emission spectrum of the emissive layer. In otherembodiments, the patterning of the enhancement layer can be configuredto target one wavelength or a subset of wavelengths within the spectrumof the wavelengths emitted from the emissive layer. This is also truefor the outcoupling layer disclosed herein.

The patterned enhancement layer can be fabricated in a number of ways.The most precise methods include: photolithography, imprint lithography,or electron beam lithography. Quasiperiodicity may be achieved throughdepositing on, or templating the enhancement layer with, aself-assembled layer. Quasiperiodic or random patterning may be achievedthrough roughening the substrate of the OLED device to add texture tothe enhancement layer. Any of these methods may be used to pattern asolid metallic film to achieve an optically active metamaterialenhancement layer. The enhancement layer may either be patterned whileon the substrate itself with the OLED deposited directly on top of thepatterned enhancement layer or the enhancement layer may be patterned onan alternative substrate and then place on the OLED device. Placementmethods include stamping, wafer bonding, wet transfer, and ultrasonicbonding.

Patterning the enhancement layer to create a resonant plasmonic effectmay be accomplished by two dimensional (2D) or three dimensional (3D)patterning. Periodically patterned enhancement layers may also bereferred to as gratings. In a 2D grating, the structural featuresforming the grating, the wavelength-sized or sub-wavelength-sizedfeatures, are arranged in a periodic pattern that is uniform along onedirection (i.e., in x-direction or y-direction as labeled in FIG. 13A)in the plane of the enhancement layer. The top-down views of FIGS. 13Aand 13B illustrate examples. In a 3D grating, the enhancement layer isformed of two sets of gratings, wherein each set of gratings is orientedin a different direction. In some preferred embodiments, the structuralfeatures in the two sets of gratings are oriented orthogonal to eachother. The periodic pattern formed by the wavelength-sized orsub-wavelength-sized features in each of the gratings in the two sets ofgratings that make up a 3D grating can be uniform or non-uniform alongone direction in the plane of the enhancement layer. mensions, i.e. inx-direction and y-direction in the plane of the enhancement layer. InFIGS. 13A and 13B, the dark regions and the white regions illustrate thetwo different materials forming the enhancement layer. As understood bythose skilled in the art, either material (i.e., the dark regions or thewhite regions in the figures) can be considered to be thewavelength-sized or sub-wavelength-sized features forming the grating.In other embodiments, where the enhancement layer comprises a stack ofmultiple layers (such as unit cells as described in conjunction withFIGS. 5 and 6), one or more of the layers in the stack can be a 2D or a3D patterned grating layers.

Lateral patterning of the enhancement layer as specified above may alsobe used to tune the spectral width, frequency, and loss of theenhancement layers plasmonic modes. A narrow plasmonic enhancementallows for color tuning of the emitter. A less lossy enhancement layerincreases the efficiency of the OLED device. A resonant plasmonic modein the enhancement layer may increase the enhancement to the rateconstant of emission of the emitter.

The enhancement layer can be formed as at least one set of gratings. In2D patterned grating embodiments where one layer has one gratingpattern, the grating can have a periodic pattern, wherein thewavelength-sized or sub-wavelength-sized features are arranged uniformlyalong one direction. The wavelength-sized or sub-wavelength-sizedfeatures can be arranged with a pitch of 100-2000 nm with a 10-90% dutycycle, and more preferably 20-1000 nm with a 30-70% duty cycle. Thepattern may be composed of lines or holes in the enhancement layer.However the pattern does not need to be symmetric. It could be locallypatterned over the distance of 1 micrometer and then have no patteringfor several micrometers before repeating the pattering again.

In 3D patterned grating embodiments, the enhancement layer is formed oftwo sets of gratings, wherein in each set of gratings, thewavelength-sized or sub-wavelength-sized features are arrangednon-uniformly along one direction with a pitch of 100-2000 nm with a10-90% duty cycle, wherein each set of gratings is oriented in differentdirection. The two sets of gratings can be oriented orthogonal to eachother. The wavelength-sized or sub-wavelength-sized features can bearranged with a pitch of 100-2000 nm with a 10-90% duty cycle inx-direction and y-direction. Preferably, the wavelength-sized orsub-wavelength-sized features can be arranged with a pitch of 20-1000 nmwith a 30-70% duty cycle in both x and y-directions. There is also norequirement on symmetry for 3D patterning.

According to an aspect of the disclosure, whether the enhancement layeris provided as a film layer or at least one set of gratings, theenhancement layer can be formed as vertically stacked repeated unitcells. All of the unit cells in the stack can be the same or each of theunit cells in the stack can have different material composition.Preferably, an enhancement layer can have up to 10 unit cells and morepreferably up to 5 unit cells. In an embodiment that has one unit cell,such unit cell can be a single layer of plasmonic material whether it bea solid film layer or a grating layer. It should be noted that this isdifferent from a DBR in the fact that more unit cells in the enhancementlayer does not necessarily represent greater performance.

In embodiments having multiple layer unit cells, the unit cells may havesubcomponents, as shown in FIG. 5. FIG. 5 features an enhancement layer300 comprised of three unit cells 310, 320, and 330, where each unitcell has two subcomponent layers, S1 and S2.

In some embodiments, each subcomponent layers can be further formed ofmultiple layers of materials. Examples of such architecture are shown inFIG. 6. In the unit cell 310, first subcomponent S1 is a metallic layer311 and second subcomponent S2 is a dielectric layer 312. In the unitcell 320, first subcomponent S1 comprises a first metallic layer 321 anda second metallic layer 322, and second subcomponent S2 is a dielectriclayer 323. In the unit cell 330, first subcomponent S1 is a metalliclayer 331 and second subcomponent S2 comprises a first dielectric layer332 and a second dielectric layer 333. In the unit cell 340, firstsubcomponent S1 comprises a first metallic layer 341 and a secondmetallic layer 342. Second subcomponent S2 comprises a first dielectriclayer 343 and a second dielectric layer 344. The plasmonic material canbe a metal selected from the group consisting of Ag, Au, Al, Pt, andalloys of any combination of these materials. The plasmonic material canalso be conducting doped oxides (examples include In—Ga—ZnO and In—Snoxide), or doped nitrides. Most preferably the plasmonic material is Ag.

In some embodiments, the enhancement layer has an imaginary component ofthe refractive index greater than 1 over for some part of the wavelengthspectrum from 400-750 nm.

The enhancement layer may be deposited by a number of processingtechniques including electron beam evaporation, thermal evaporation,atomic layer deposition, sputtering, and various chemical vapordeposition techniques. The dielectric layer can include small organicmolecules, polymers, wide bandgap oxides (SiO2, TiO2, Al2O3, etc.),insulating nitrides, and undoped semiconductors (Si and Ge for example).The real part of the refractive index of these materials can span 1.3 to4.1. The imaginary component may be less than 1 over the wavelengthrange of 400 to 750 nm. The dielectric layers may be deposited bythermal evaporation, ink jet printing, organic vapor jet printing, spincoating, doctor blading, the Langmuir-Blodgett technique, pulsed laserdeposition, sputtering, and various chemical vapor deposition methodsincluding atomic layer deposition. Optically active metamaterials canalso be made by patterned grooves in solid metallic films. The filmswould be deposited by any of the methods cited above.

Referring to an example shown in FIG. 7, in some embodiments, theenhancement layer is a stack 400 comprising a film of plasmonic material410 and a film of an adhesion material 412. The plasmonic material is ametal selected from the group consisting of Ag, Au, Al, Pt, and alloysof any combination of these materials. The adhesion material is selectedfrom the group consisting of Ni, Ti, Cr, Au, Ge, Si, and alloys of anycombination of these materials. Preferably the adhesion material is Ni,Ti, or Ge and more preferably Ge. In one preferred embodiment, theplasmonic material is Ag and the adhesion material is Ge.

Preferably, the layer of plasmonic material has a thickness of 0.2 to 50nm and the layer of adhesion material has a thickness of 0.1 to 10 nm.More preferably, the layer of adhesion material has a thickness of 0.2to 5 nm. More preferred thicknesses of the plasmonic material depend onthe exact OLED structure but typically range from 5 to 30 nm. When usingthis preferred structure the RMS surface roughness of the enhancementlayer measured over a 2 μm by 2 μm area should be between 0 to 5 nm;more preferably between 0 and 2 nm. The resistivity of the Ag layershould fall between 0.1 to 100 ohms per square, more preferably 0.5 to20 ohms per square. The transparency of the preferred structure betweenthe wavelength range of 400 to 800 nm falls between 40 to 100%; morepreferably, between 60 to 95%.

The enhancement layer can be incorporated into an OLED in various ways.FIGS. 8A-8C and FIG. 9 demonstrate examples of top and bottom emittingOLEDs which have the enhancement layer. The OLED's emission can bemonochrome, multi-colored, or white. FIGS. 8A-8C show some examples ofOLEDs according to the present disclosure in which both anode andcathode contace layers are provided in addition to the enhancementlayers. FIG. 8A shows an example of a top-emitting OLED 410 comprising,in the positional order, a substrate 411, a first electrode 412 (ananode in this case), organic EML layer 413, a second electrode 414 (acathode in this case), an enhancement layer 415, an intervening layer416, and an outcoupling layer 417. FIG. 8B shows an example of abottom-emitting OLED 420 comprising, in the positional order, atransparent substrate 421, an outcoupling layer 422, an interveninglayer 423, an enhancement layer 424, an anode 425, an organic EML layer426, and a cathode 427. FIG. 8C shows a two-way emitting OLED 430comprising, in the positional order, a transparent substrate 431, anoutcoupling layer 432, an intervening layer 433, a first enhancementlayer 434, an anode 435, an organic EML layer 436, a cathode 437, asecond enhancement layer 438, a second intervening layer 439A, and asecond outcoupling layer 439B.

As disclosed herein, in some embodiments of the OLEDs according to thepresent disclosure, the OLEDs can be configured so that the enhancementlayers are also the contact layers. FIGS. 9A-9E illustrate suchexamples. FIG. 9A shows a bottom-emitting OLED 510 comprising, in thepositional order, a transparent substrate 511, an outcoupling layer 512,an intervening layer 513, an enhancement layer 514, one or more optionalfunctional layers 515, an organic EML 516, and a cathode 517. In theOLED 510, the enhancement layer 514 is used as the anode contact layer.FIG. 9B shows a top-emitting OLED 520 comprising, in the positionalorder, a substrate 521, an anode 522, an organic EML 523, one or moreoptional functional layers 524, an enhancement layer 525, an interveninglayer 526, and an outcoupling layer 527. In the OLED 520, theenhancement layer 525 is used as the cathode contact layer. FIG. 9Cshows a two-way emitting OLED 530 comprising, in the positional order, atransparent substrate 531, a first outcoupling layer 532, a firstintervening layer 533, a first enhancement layer 534, an anode 535, anorganic EML 536, one or more optional functional layers 537, a secondenhancement layer 538, a second intervening layer 539A, and a secondoutcoupling layer 539B. In the OLED 530, the second enhancement layer538 is used as the cathode contact layer. FIG. 9D shows a two-wayemitting OLED 540 comprising, in the positional order, a transparentsubstrate 541, a first outcoupling layer 542, a first intervening layer543, a first enhancement layer 544, one or more optional functionallayers 545, an organic EML 546, a cathode 547, a second enhancementlayer 548, a second intervening layer 549A, and a second outcouplinglayer 549B. In the OLED 540, the first enhancement layer 544 is used asthe anode contact layer. FIG. 9E shows another two-way emitting OLED 550comprising, in the positional order, a transparent substrate 551, afirst outcoupling layer 552, a first intervening layer 553, a firstenhancement layer 554, a first set of one or more optional functionallayers 555, an organic EML 556, a second set of one or more optionalfunctional layers 557, a second enhancement layer 558, a secondintervening layer 559A, and a second outcoupling layer 559B. In the OLED540, the first enhancement layer 554 and the second enhancement layer558 are used as the anode contact layer and the cathode contact layer,respectively.

In the OLED examples of FIGS. 9A-9E, the various sets of one or moreoptional functional layers 515, 524, 537, 545, 555, 557, and the contactlayers 535 and 547 that are positioned between an enhancement layer andan organic EML layer are configured to have the appropriate thickness tomeet the requirement that the corresponding enhancement layer is no morethan the threshold distance away from the respective organic EML layer.

FIGS. 8A, 8B, and 9A-9E illustrate embodiments of devices that includeintervening layers. However, as discussed throughout, embodimentswithout the intervening layers are also within the scope of the presentdisclosure.

An OLED's contact materials are often chosen so that the Fermi levelinjects charges into the device while the metallic layers of theenhancement layer are chosen for their optical properties. However,using hole and electron injection layers can facilitate charge injectionallowing the enhancement layer to act as a contact layer for the OLED.Thus, using the enhancement layer as a contact is not a requirement ofthe layer but an opportunity to reduce the complexity of manufacturing.

If the enhancement layer is not used as a contact, the OLED contactlayer is a conductive medium typically composed of either a transparentconducting oxide (TCO) or a metal film. Typical TCO thicknesses rangefrom 50 to 200 nm, with more preferred thicknesses between 80 to 150 nmwhen the TCO is not between the enhancement layer and the EML. When theTCO is between the enhancement layer and the EML the total thicknessmust be less than the threshold thickness. Metal contact layerstypically vary between 7 to 300 nm in thickness when not between theenhancement layer and the EML.

If the enhancement layer is chosen as a contact, it could be implementedin two different ways: either the entire structure is the contact or asub-component is. If a partially conductive dielectric material isincluded as a subcomponent layer (given that it meets the opticalrequirements) the entire enhancement layer is conductive and acts as thecontact. In contrast, the enhancement layer can include a metallic layersubcomponent which could be the contact. This includes using either thefirst metallic layer within the enhancement layer or the last metalliclayer in the enhancement layer depending on the exact OLED architecture(top or bottom emitting). FIGS. 9A-9E highlight various devicearchitectures which demonstrate the placement of the enhancement layerwithin an OLED device when an enhancement layer is used as at least oneof the OLEDs contacts.

The increased emission rate constant of the OLED emitter is stronglydependent on the distance of the emitter from the enhancement layer. Toachieve better OLED performance the preferred distance from theenhancement layer to the EML is not greater than the threshold distanceand is as small as possible within the limits discussed herein. Thetypical threshold distance for a phosphorescent molecule is less than100 nm, and more typically less than 60 nm. Achieving the short distancebetween the EML to the enhancement layers may require adjusting the OLEDarchitecture by controlling the thickness of a contact layer or otherone or more functional layers that may be provided between the organicEML layer and an enhancement layer. FIG. 10 and FIG. 11 show examples ofOLED device architectures showing some of such one or more functionallayers that may be used to achieve the desired distance between theenhancement layers and the organic EML layer.

FIG. 10 provides an illustration of a bottom-emitting OLED 600. The OLED600 comprises, in the positional order, a transparent glass substrate601, an outcoupling layer 602 disposed over the substrate, anintervening layer 603, an enhancement layer 604, a first set of one ormore optional functional layers 605, an organic EML layer 606, a secondset of one or more optional functional layers 607, and a cathode contactlayer 608. According to an aspect of the present disclosure, theenhancement layer 604 in this example is formed of two subcomponentlayers: adhesion layer 604 a and a plasmonic material film layer 604 b.The first set of one or more optional functional layers 605 may beselected from a protective TCO layer 605 a, a hole injection layer 605b, a hole transport layer 605 c, and electron blocking layer 605 d. Thesecond set of one or more functional layers 607 may be selected from ahole blocking layer 607 a, an electron transport layer 607 b, and anelectron injection layer 607 c.

FIG. 11 provides an illustration of a top-emitting OLED 700. The OLED700 comprises, in the positional order, a substrate 701, an anode 702, afirst set of one or more optional functional layers 703, an organic EMLlayer 704, a second set of one or more optional functional layers 705,an enhancement layer 706, an intervening layer 707, and an outcouplinglayer 708. The first set of one or more optional functional layers 703may be selected from a hole injection layer 703 a, a hole transportlayer 703 b, and an electron blocking layer 703 c. The second set of oneor more optional functional layers 705 may be selected from a holeblocking layer 705 a, an electron transport layer 705 b, an electroninjection layer 705 c. The enhancement layer 706 comprises a stack ofthree unit cells 706 a, 706 b, and 706 c, wherein each of the unit cellsconsist of a Ag as the plasmonic material film 716 a, and a hostmaterial film 716 b. The detailed discussion of such stacked unit cellstructure is provided above in connection with the discussion of FIGS. 5and 6.

When using the enhancement layer as a contact for the OLED, it may bebeneficial to use nontraditional materials or no materials at all as thehole or electron injection layer in order to bring the molecules in theexcited state in the EML of the OLED closer to the enhancement layer. Inaddition to the materials defined earlier, we define a hole injectionmaterial is any material with a HOMO lower than or equal to the dopantin the OLED. This sets a preferred range for the HIL HOMO from −8 eV to−4.7 eV. Importantly, the emitter material itself can act as the holeinjection layer. Similarly, an electron injection material will be amaterial with a LUMO level slightly below to above the LUMO level of theOLED dopant. The preferred range for the EIL material's LUMO is from −4eV to −1.5 eV. Charge injection may also be accomplished by the dopantitself either as a neat layer or highly doped in a host.

When using the enhancement layer, the layers between the enhancementlayer and the EML are preferably thinner than the threshold distance toachieve the best result. For example, when using the enhancement layernear or as the anode, the HIL and HTL layers should be quite thin.Improving device yield or manufacturability of the OLED may requireincreasing the overall thickness of the OLED.

In theory, the OLED can be of arbitrary thickness. However, for thickerOLEDs it is expected that the operational voltage will increase due toresistive losses in transport of charge through the charge transportinglayers. Thus, the preferred OLED thickness range for a single stack OLEDis from 10 to 500 nm, more preferably from 20 to 300 nm. For OLEDs withmultiple EMLs the thicknesses scales with the number of EMLs. Forexample, a 2 EML device has a preferred OLED thickness range from 20 to1000 nm, more preferably from 40 to 600 nm.

When using an enhancement layer in an OLED the layers opposite theenhancement layer will have minimal effect on the emitter's properties,especially if excitons are confined within the EML. However, there aresecond order effects that can be maximized to improve the performance ofthe enhancement layer OLED.

Changes occur in the density of states that the enhancement layercreates for the emitting molecule when the total thickness of the OLEDis changed. FIG. 12 is a plot of the calculated enhancement of theemitter's emission rate constant in the OLED relative to the emissionrate constantly in air as a function of the OLEDs total thickness andthe emitter's peak emission wavelength. The emission rate constant isthe total decay rate constant which is the sum of non-radiative andradiative decay but is dominated by non-radiative decay for these devicestructures. The enhancement layer-to-emitter distance was kept constant.There are nodes of increased performance (see emitter peak emission of530 nm) about every 150 nm of OLED thickness. However, the change in therate constant enhancement is ˜10 to 30% higher at a peak than in thetrough, changes that are smaller than moving the light emitter fartherfrom the metallic film.

The in-plane momentum of the surface plasmon in the enhancement layerdepends on the refractive index of the layers bordering the enhancementlayer. Inserting an intervening layer between the enhancement layer andthe outcoupling layer can tune the wavelength of light that isoutcoupled to free space by an outcoupling layer of fixed periodicity.The index of refraction of the material between the enhancement layerand the outcoupling layer also changes the fraction of energy that flowsbetween the two as it modifies the total mode overlap. The real part ofthe index of refraction of the intervening layers is between 1.1 to 4.0and more preferably between 1.3 to 2.4. The intervening layer may be adielectric material, a semiconductor material, a metal, or anycombination thereof and has a thickness between 1-20 nm, more preferablybetween 1-10 nm.

The benefit of using the enhancement layer to increasing the EQE of anOLED may be best realized by using the enhancement layer in combinationwith an outcoupling layer to increase the number of photons directedinto free-space. The increase in excited state decay rate constant ofthe light emitter in the EML is due to the excited state energytransferring to the nonradiative modes of the surface plasmon of theenhancement layer. Once the energy is transferred to the surfaceplasmon, the surface plasmon cannot couple all the energy to free-space.The outcoupling layer will remove the captured energy from theenhancement layer and couple that light to free-space.

Realizing an enhancement layer with a coupling layer could result an inOLED with an EQE greater than 40% and theoretically have an EQE of 100%.An EQE in excess of ˜43% exceeds the conventional limit and can beachieved even with a high number of vertically oriented emitters. Thecloser the light emitting material is to the enhancement layer thesmaller the impact of molecular orientation on the performance of theenhancement layer. This is in contrast to the typical OLED structure. Ifthe outcoupling layer is place directly on top of the substrate, ratherthan on top of the OLED, then the outcoupling layer can be fabricated onthe substrate prior to OLED deposition. This allows high temperatureprocess for any of the materials in the outcoupling layer. It alsoallows high resolution patterning of the outcoupling layer using photo,interference, nanoimprint, e-beam, ion beam, focused ion beam, and otherlithography techniques that would normally destroy the organic materialscontained within the OLED.

According to an embodiment, the outcoupling layer can be comprised of aninsulating material, a semiconducting material, a metal, or anycombination thereof. The outcoupling layer can consists of two materialsof differing refractive index along a plane parallel to the enhancementlayer. Preferably, the two materials have different refractive index,wherein the difference in the real part of the refractive index isbetween 0.1 to 3.0, and more preferably between 0.3 to 3.0.

The outcoupling layer may be patterned to increase the efficiency ofscattering light from the non-radiative mode of the enhancement layer.This patterning may be periodic, quasi-periodic, or random. Preferredperiodicities are on the order of multiples of the wavelength (m*λ,where m is an integer starting from 1 and λ is the wavelength of lightin that material) of the light in the medium of the outcoupling layer.More preferred periodicity is on the order of or smaller than thewavelength of the light in the medium of the outcoupling layer.Scattering of light in the enhancement layer may occur through Bragg orMie scattering. The outcoupling layer may be composed of insulating,semiconducting, or metallic materials or any combination of these typesof materials. The preferred case are two materials which have arefractive index constant contrast. Some examples are high refractiveindex material that are transparent such as TiO2, ZrO2, diamond, Si3N4,ZnO, high refractive index glasses (which usually have these materialsas components); high refractive index, light absorbing materials: Group4 and 3-5 semiconductors like Si, Ge, GaAs, GaP; and low refractiveindex materials: SiO2, most glasses, polymers, organic molecules(˜1.6-1.8), MgF2, LiF, air or vacuum.

Preferred examples of highly periodic structures is a linear gratingpatterned from two materials. Bragg scattering occurs due to the highlyperiodic interfaces from the two materials of differing dielectricconstant in the x-y plane. The linear grating could be periodic in 2D or3D. The difference in the real part of the refractive index between thetwo materials should be between 0.1 to 3. More preferably from 0.3 to 3.For 2D patterning, the preferred pitch is 10-6000 nm with a 10-90% dutycycle, more preferably a 20-1000 pitch with a 30-70% duty cycle. For 3Dpatterning the preferred pattern is a pitch of 10-6000 nm with a 10-90%duty cycle, more preferably a 20-1000 pitch with a 30-70% duty cycle inthe x-direction and a 10-6000 nm pitch with a 10-90% duty cycle, morepreferably a 20-1000 pitch with a 30-70% duty cycle, in the y-direction.A preferred example of the linear grating is when one of the gratingmaterials is metallic.

The periodicity of the outcoupling layer not only sets the color oflight that is scattered out from the enhancement film, it also sets theangle(s) at which that light will be coupled to air. Changing either therefractive index or period of the outcoupling layer will change the‘angular dependence’ meaning the intensity of the OLED as a function ofangle. Different outcoupling structures may be used depending on theangular dependence of the emission that is desired. Some outcouplingstructures that maximize the amount of energy extracted from theenhancement layer might not give the desired angular distribution oflight into free space. In this case, the OLED device may have a diffuserplaced in front of the pixel to modify the angular dependence of theOLED emission to the desired shape. It is also possible to tuning theoutcoupling layer materials and pattering to enhance the amount of lightdirected into free-space modes.

The polarization of the emission can be tuned using the outcouplinglayer. Varying the dimensionality and periodicity of the outcouplinglayer can select a type of polarization that is preferentiallyoutcoupled to air.

An embodiment of the outcoupling layer which may be periodic,quasi-periodic, or random is the suspension of micro- or nano-particleswithin a host matrix. The periodicity of the particles can be randomlythroughout the host matrix, however, the size distribution of theparticles may be very tight (thus highly periodic). The micro- ornano-particles may be spheres, rods, squares, or other threedimensionally shaped materials having sharp edges and with a refractiveindex different from the host material. All high refractive indexmaterials disclosed herein can be used for the outcoupling layerincluding metals. The host for the scattering particles may be adielectric, metallic, or semiconducting. The loading of the micro- ornanoparticle may span from 5 to 95% by weight. The preferred size of themicro- or nano-particle will have at least one dimension on the order ofwavelength of visible light in that medium or smaller, typically between50 nm to 800 nm. The refractive index contrast between the host and thescattering medium is important in tuning the efficiency of scattering.The preferred absolute value of the difference is between 0.1 to 3.0,more preferably from 0.4 to 3.0. Color tuning of the outcoupled light ispossible by varying the size and fill fraction of the scatteringparticles. A preferred embodiment of outcoupling layer is metallicmicro- or nano-particles which have sharp edges.

It is expected that the various grating techniques can achieve a colorshift of up to 50 nm from the intrinsic peak of the molecule while notsacrificing the amount of light outcoupled to air. The gratings can alsoachieve narrowing of the intrinsic molecules emission spectrum. Whendesigned to reduce the FWHM the FWHM will be between 10 to 50 nm, withmore preferred outcoupling layers achieving a 10 to 30 nm FWHM.

In some embodiments, the outcoupling layer is at least one set ofgratings formed of wavelength-sized features that are arrangedperiodically, quasi-periodically, or randomly, or sub-wavelength-sizedfeatures that are arranged periodically, quasi-periodically, orrandomly. In one preferred embodiment, the wavelength-sized features andthe sub-wavelength-sized features have sharp edges.

In some embodiments, the grating is a linearly patterned grating havinga spacing pitch and is formed of two alternating materials. The linearpattern can be in 2D or 3D. In a 2D embodiment, each grating materialforms wavelength-sized or sub-wavelength-sized elongated features thatare arranged uniformly along one direction with a pitch of 100-2000 nmwith a 10-90% duty cycle, and more preferably with a pitch of 20-1000 nmwith a duty cycle of 30-70%. Dielectric materials can be used for thetwo grating materials.

For the 3D linear pattern embodiment, the outcoupling layer is formed oftwo sets of gratings, each set of gratings formed of two materials,wherein in each set of gratings, each material forms wavelength-sized orsub-wavelength-sized features that are arranged nonuniformly along onedirection with a pitch of 100-2000 nm with a 10-90% duty cycle, whereineach set of gratings is oriented in different direction. Each set ofgratings can be oriented orthogonal to each other, i.e., in x-directionand y-direction. Dielectric materials can be used for the two gratingmaterials.

In some embodiments, the outcoupling layer is a bullseye grating havinga set of concentric rings with well defined spacing. The preferred pitchor periodicity of the grating is 100-2000 nm and comprises a dielectricmaterial having a refractive index between 1.3 and 4, and wherein spacebetween the grating is filled with any material with a real part of therefractive index between 0.1 to 4.

In some embodiments, a quasi-periodic outcoupling layer is a chirpedgrating. In a chirped grating, the periodicity varies as a function ofdistance across one or two dimensions. The preferred structure for thechirped grating outcoupling layer has a fundamental period of between 10to 2000 nm and increases by 10-60% per period, and wherein theoutcoupling layer comprises a dielectric material having a refractiveindex between 1.3 and 4.

In some embodiments, the outcoupling layer comprises a plurality ofparticles in a host material, wherein the plurality of particles have aphysical dimension smaller than the wavelength of the light beingemitted to free space. Preferably, the particles have a physicaldimension in the range of 50-800 nm and more preferably 200-800 nm. Insome embodiments, the plurality of particles are non-sphericalnanoparticles having three dimensional shape such as rods, cubes, andpolyhedron. The plurality of particles can be a dielectric material,semiconductor material, or a metal. If the particles are metallic mostdielectric or semiconducting material can be used as the host material.If the particles are dielectric or semiconducting, the host material ispreferably another dielectric or semiconducting material having a largerrefractive index is preferred.

In some embodiments, the outcoupling layer comprises a patternedmetallic film.

In some embodiments, the enhancement layer is a second electrode.

In some embodiments, the device further comprises a second electrodedisposed between the intervening layer and the outcoupling layer.

The enhancement layer does not require optical interference to maximizethe fraction of light outcoupled to free space. Since the enhancementlayer's Fermi level can be independent of the contact's Fermi level, theOLED can be inverted without any additional fabrication constraints.This remains true for the preferred enhancement layer of Ge/Ag. If theGe/Ag enhancement layer were to replace the ITO or IZO anode in atypical OLED device with an Al cathode, then the two contacts are nearlyat the same Fermi level. Use of EIL and HIL materials enable efficientcharge injection so that the organic layers can be inverted between theenhancement layer and the Al layer.

Typical OLEDs depend on one reflective contact to increase the amount ofemission coupled to free space. When the enhancement layer is used withan outcoupling layer all the energy of the dipole could be coupled tothe front free space mode without a mirror on the backside of the OLED.This allows the entire OLED to be transparent and for it to be a 1-sideddisplay, not wasting any energy on emission towards the backside of thedisplay.

When the enhancement layer is combined with the outcoupling layer theentire structure may not need a front polarizer to prevent ambient lightfrom being reflected to the user. To achieve this outcome, theoutcoupling layer must transmit incident radiation through the OLED toan absorbing medium while still outcoupling the emission that isoriginating from the enhancement layer.

When the enhancement layer is used with an outcoupling layer all theenergy of the excited state could be coupled to the front free spacemode. Preventing the reflection of ambient light can occur in twovarious methods. The first is to build a transparent OLED. Ambient lightis transmitted through the OLED and absorbed on the back of the OLEDwhich is coated with an absorbing medium. The second method is to absorblight on a ‘dark cathode.’ A dark cathode is a conductive material thatis highly absorbing. This can be accomplished in a number of waysincluding texturing of reflective metals or the use of conductive butabsorbing materials (highly doped semiconductors).

The enhancement layer either with or without the outcoupling layer canbe implemented on flexible substrates. Flexible substrates include: thinglass, polymer sheets, thin silicon, metal sheets, and paper sheets. Asthe typical enhancement layer will typically have a thickness less than1 micrometer, and is preferentially thinner than 100 nm, the entire OLEDwill be flexible while still achieving the benefits to efficiency andlifetime.

Growth of the enhancement or outcoupling layer on any of the flexiblesubstrates may require a planarization layer to smooth the roughness ofthe substrates before deposition. Alternatively, the outcoupling orenhancement layer could be grown in a conformal processing step usingtechniques like high pressure sputtering, spray coatings, atomic layerdeposition, chemical vapor deposition, or plasma enhanced chemical vapordeposition. Techniques that are typically considered non-conformal, suchas vacuum thermal evaporation may be conformal enough if they are usedwith an off-axis source and off-axis rotation. The rms surface roughnessof the planarization layer should be less than 25 nm, preferably lessthan 10 nm, more preferably less than 5 nm, most preferentially lessthan <3 nm.

The layered structure of the enhancement layer could also benefitflexible OLEDs by acting as an oxygen and water diffusion barrier.

As the enhancement layer effect is strongly dependent on the distance ofthe emitter from the enhancement layer, it is possible to use theenhancement layer to increase the performance of a mixed emitter OLED. Apreferred embodiment is a white OLED with a blue light emitting materialand at least one lower energy light emitting material. In the preferredembodiment the enhancement layer is closest to the blue light emittingmaterial as it is least durable light emitting material. In thisembodiment the enhancement layer must be at least semitransparent tolower energy emission. Additionally, if any outcoupling layer is used,it must also either be transparent to lower energy emission or outcouplelower energy emission as well as the blue emission from the blue lightemitting material. When using the enhancement layer with a white OLED,the white OLED can be either a single EML device or a stack structure.The emitter(s) in the EML closest to the enhancement layer will have anincreased performance. In the stacked structure, the number andthicknesses of the additional EMLs can be tuned completely independentlyof the enhancement layer.

In one embodiment of an OLED with an enhancement layer the display ismanufactured with no fine metal masking steps. In this embodiment, thecolor of each subpixel is determined by one of two methods. The first isto vary the periodicity and refractive indices of the outcoupling layerunder each sub-pixel. Each sub-pixel has a periodicity of theoutcoupling layer that is tuned to outcouple light at the desiredfrequency. The second method is to keep the periodicity and refractiveindices of the outcoupling layer identical for each sub-pixel but tovary the refractive index of the intervening layer for each sub-pixel.For bottom emitting enhancement layer OLEDs, the pattering of theoutcoupling layer or the intervening is completed on the substratebefore OLED deposition. A white OLED with a single EML composed ofmultiple color light emitting materials is uniformly fabricated over theoutcoupling layer achieving a display with no fine metal masks. Thismanufacturing technique is similar to the white plus color filter bottomemitting OLED display in that it uses a blanket OLED deposition. Eachpixel is defined by the combination of an outcoupling layer and theenhancement layer. Since energy is transferred from the light emittingmaterial to the non-radiative modes of the enhancement layer, no lightwill be emitted unless the outcoupling layer is also present. Thus, theblanket OLED will not illuminate unless an outcoupling layer is present,allowing for fabrication of a display with a high value of pixels perinch with a blanket OLED deposition and no metal masks.

It is also possible to design a display with R,G,B pixels or sub-pixelswith very low resolution masking. The advantage of this technology isthat the driving voltage of the red and green pixels is not determinedby the blue emitter as is the case for the no shadow masking case. Inthis case, the operating power of the display will be reduced.

The enhancement layer can be designed to emit no radiation unless thereis an outcoupling layer built in conjunction with it. Thus, very roughresolution masking of different color OLEDs can be converted into highresolution pixels by patterning the outcoupling layer with highresolution. This high resolution patterning can occur using a number offabrication techniques and is not limited by temperature or the use ofsolvent as the substrate may not have organic materials disposed on itwhen it is patterned.

After fine patterning of the outcoupling layer, low resolution masks forR,G,B pixels can be used to make R, G, and B OLEDs. Regions on whichOLED material is deposited but which do not have an outcoupling layerwill not emit light meaning the display will have the resolution of thefinely patterned outcoupling layer not the lower resolution of theshadow masks.

The emission spectrum from the combination of the enhancement layer andthe outcoupling layer can be designed to be directional. In heads updisplay and displays for virtual reality a display with directionalityemission can be beneficial. The increased directionality of emission canbe used to only project light on to the eye when the person is lookingat one direction relative to the OLED. In one embodiment an OLED devicewith an enhancement layer and outcoupling layer will be used for a headsup display or in a virtual reality device.

Coupling the enhancement layer with outcoupling layer could result in avery bright, highly directional OLED. Such an OLED is well designed foruse as an automotive taillight.

The enhancement layer increases the excited state decay rate constant ofthe emitter within the OLED. Near infrared phosphors suffer lowphotoluminescent quantum yields due to the intrinsic non-radiative decayrate constant being much larger than the radiative decay rate constant.If the enhancement layer increases the non-radiative decay constant to avalue much larger than the intrinsic non-radiative decay constant itwill create an efficient near infrared OLED.

An enhancement layer which features a resonant plasmonic mode maysupport a quality factor large enough to undergo stimulated emission. Ifthe stimulated emission is pumped by injection of charge from thecontacts the end result would be an electrically pumped laser using anorganic semiconductor.

In some embodiments, the method of the present disclosure furthercomprises providing an intervening layer between the enhancement layerand the outcoupling layer, wherein the intervening layer has a thicknessless than 50 nm, preferably less than 20 nm thick, and more preferablyhas a thickness between 1-10 nm. The intervening layer can be adielectric material or a semiconducting material. The refractive indexof the intervening layer is preferably between 0.1 to 4.0. In onepreferred embodiment, the refractive index of the intervening layer isbetween 1.4 to 4.0. The intervening layer has an index of refractionwhose real part is 1.1 to 4.0, and more preferably between 1.3 to 2.4.

According to another aspect of the present disclosure, an enhanced OLEDdevice is disclosed. The OLED comprises: a substrate; a first electrode;an organic emissive layer comprising an organic emissive materialdisposed over the electrode; an enhancement layer, comprising aplasmonic material exhibiting surface plasmon resonance thatnon-radiatively couples to the organic emissive material and transferexciton energy from the organic emissive material to non-radiative modeof surface plasmon polaritons, disposed over the organic emissive layeropposite from the first electrode, wherein the enhancement layer isprovided no more than a threshold distance away from the organicemissive layer, wherein the organic emissive material has a totalnon-radiative decay rate constant and a total radiative decay rateconstant due to the presence of the enhancement layer and the thresholddistance is where the total non-radiative decay rate constant is equalto the total radiative decay rate constant; and an outcoupling layerdisposed over the enhancement layer, wherein the outcoupling layerscatters the energy from the surface plasmon polaritons as photons tofree space. In some embodiments of this enhanced OLED device, anintervening layer is further disposed between the enhancement layer andthe outcoupling layer, wherein the intervening layer has a thicknessbetween 1-10 nm and a refractive index from 0.1 to 4.0. The interveninglayer has the characteristics as described above.

In some embodiments, the substrate is transparent and is disposedadjacent to the outcoupling layer opposite from the enhancement layer(i.e. a bottom-emitting device), or the substrate is disposed adjacentto the first electrode opposite from the organic emissive layer (i.e., atop-emitting device).

In some embodiments, an OLED device can have more than one enhancementlayer. According to some embodiments, the OLED device comprises asubstrate, wherein the substrate can be transparent; a first outcouplinglayer disposed over the substrate;

-   -   a first enhancement layer disposed on the first outcoupling        layer;    -   an organic emissive layer comprising an organic emissive        material disposed over the first enhancement layer,        -   wherein the first enhancement layer comprising a first            plasmonic material exhibiting surface plasmon resonance that            non-radiatively couple to the organic emissive material and            transfer excited state energy from the organic emissive            material to non-radiative mode of surface plasmon            polaritons,        -   wherein the first enhancement layer is provided no more than            a threshold distance away from the organic emissive layer;    -   a second enhancement layer disposed over the organic emissive        layer, the second enhancement layer comprising a second        plasmonic material exhibiting surface plasmon resonance that        non-radiatively couple to the organic emissive material and        transfer excited state energy from the emissive material to        non-radiative mode of surface plasmon polaritons,        -   wherein the second enhancement layer is provided no more            than the threshold distance away from the organic emissive            layer,        -   wherein the organic emissive material has a total            non-radiative decay rate constant and a total radiative            decay rate constant due to the presence of the first and            second enhancement layers and the threshold distance is            where the total non-radiative decay rate constant is equal            to the total radiative decay rate constant; and    -   a second outcoupling layer disposed over the second enhancement        layer, wherein the first and second outcoupling layers scatter        the energy from the surface plasmon polaritons as photons to        free space. In other embodiments, this OLED device further        comprises a first intervening layer disposed between the first        outcoupling layer and the first enhancement layer; and a second        intervening layer disposed between the second enhancement layer        and the second outcoupling layer.

According to another aspect of the present disclosure, methods formanufacturing plasmon OLED devices are also disclosed. A method formanufacturing a top-emitting organic light emitting device comprises:providing a substrate; depositing a first electrode; depositing anorganic emissive layer comprising an organic emissive material over thefirst electrode; depositing an enhancement layer comprising a plasmonicmaterial disposed over the organic emissive layer no more than athreshold distance away from the organic emissive layer, wherein theorganic emissive material has a total non-radiative decay rate constantand a total radiative decay rate constant due to the presence of theenhancement layer and the threshold distance is where the totalnon-radiative decay rate constant is equal to the total radiative decayrate constant; and depositing an outcoupling layer disposed over theenhancement layer. In some embodiments, the method further includesdepositing an intervening dielectric layer disposed over the enhancementlayer before depositing the outcoupling layer, wherein the interveningdielectric layer has a thickness between 1-10 nm and a refractive indexfrom 0.1 to 4.0.

In another embodiment, a method for manufacturing a bottom-emittingorganic light emitting device comprises: providing a transparentsubstrate; depositing an outcoupling layer disposed over the transparentsubstrate; depositing an enhancement layer comprising a plasmonicmaterial disposed over the outcoupling layer; depositing an organicemissive layer comprising an organic emissive material over theenhancement layer no more than a threshold distance away from theenhancement layer, wherein the organic emissive layer and theenhancement layer have a total non-radiative decay rate constant and atotal radiative decay rate constant due to the presence of theenhancement layer and the threshold distance is where the totalnon-radiative decay rate constant is equal to the total radiative decayrate constant; and depositing an electrode over the organic emissivelayer. In some embodiments, the method further includes depositing anintervening dielectric layer disposed over the outcoupling layer beforedepositing the enhancement layer, wherein the intervening dielectriclayer has a thickness between 1-10 nm and a refractive index from 0.1 to4.0.

Outcoupling of light from the enhancement layer is also modulated by theproperties of the OLED device. Coupling of the OLED side (the organicemitter layer side) of the enhancement layer to the side of theenhancement layer which points towards free space is dependent on therefractive index at each interface. Since the OLEDs' transport layerscan vary in refractive index, choice of transport layer and totalthickness may modulate the fraction of light outcoupled from theenhancement layer. Further, in top emitting enhancement layer OLEDsadding an outcoupling layer to the enhancement layer will tune theoutcoupling from that layer. The preferred values for the outcouplinglayer refractive index is 0.1 to 2.4 with a preferred thicknesses from 5to 150 nm. More preferred ranges can not be specified withoutidentifying the wavelength of emission of the emitter.

In FIG. 10, a preferred enhancement layer OLED device structure isillustrated. The OLED can be a bottom or top emitting OLED depending onthe thickness of the cathode contact. Preferred layer thicknesses arelabeled on the figure. Layers with a thickness range that includes 0 Åare optional layers, not required for the OLED to function properly whenusing the enhancement layer but featured in typical OLED devices.

Preferred structure of FIG. 6 is a top emitting OLED of FIG. 11 with anenhancement layer composed of 3 repeating of the unit cells 706 a, 706b, and 706 c. Each of the unit cells is composed of 5-15 nm of Ag as theplasmonic metallic layer 716 a and 5-20 nm of host dielectric materiallayer 716 b. The host dielectric material has a real component of therefractive index which spans 1.9 to 1.7 across the visible spectrum. Thehost dielectric material may have a real component of refractive indexfrom 1.4 to 4.0 without changing the optical properties of theenhancement layer. The imaginary component of the refractive index ofthe host dielectric should be less than 0.2 across the visible spectrum.Increasing the imaginary component of the refractive index results in aloss of outcoupling of the OLED but maintains improvements in emissionrate constant.

In one embodiment for phosphorescent blue light emitting materials theenhancement layer would be used without outcoupling layer. By carefullycontrolling the distance of the blue emitter from the enhancement layerthe number of photons lost to the non-radiative mode of the enhancementlayer can be balanced while still observing increases in the durabilityof the blue light emitting material. This may be achieved without theadditional outcoupling layer to extract energy from the enhancementlayer. Thus, a blue phosphorescent light emitting material could operatewith an EQE between 1 to 3 times the EQE of a fluorescent blue lightemitting material while achieving an increase in durability than theblue phosphorescent light emitting material without the enhancementlayer.

Experimental

We have performed initial simulations of OLEDs with enhancement layersthat demonstrate an increase in the excited state decay rate constantwhen using an enhancement layer for all light emitting molecules. Thesimulations were performed on the preferred structure, FIG. 11. Thesimulated enhancement layer OLED is compared to typical bottom emittingand top emitting devices. The intrinsic emission rate constant for theemitter was set to 1E6 s-1 for all simulations.

Optical modeling on the preferred enhancement layer structure of FIG. 6using the example enhancement layer of 11 nm Ag\11 nm HOSTA 1 nm Ag\11nm HOSTA 1 nm Ag and an outcoupling layer of 60 nm of HOST we find thatthe rate of emission of the emitter is increased to 4.7E6 to 3.6E6 s-1depending on the total OLED thickness. In contrast, the emission rate ina typical top emitting device is 1.37E6 s-1 and 1.53E6 s-1 in a bottomemitting device. The device featuring the enhancement layer representsan approximate 3 fold increase in excited state decay rate constant. Wenote that the outcoupling of the simulated structures exceeds 20% forthe enhancement layer OLEDs as well as the control bottom and topemitting OLEDs. Thus, there is no expected loss of the number of photonsthat can be extracted from the device when using the enhancement layer.Similar increases in emission rate constant can be observed forenhancement layers composed of typical manufacturing materials andmanufacturing thicknesses. For example modeling of the enhancement layerusing 15 nm of Mg:Ag 10% instead of Ag produces lifetimes of the samerange as that with Ag although the outcoupling is reduced as Mg:Ag ismore lossy than Ag.

FIGS. 14A-C demonstrate the optical modeling of the structure in FIG.11. The increase in emission rate over that of the intrinsic molecule(1E6) is clearly evident for visible emission. The excited state decayrate constant is broadly increased over the entire visible emissionrange. The enhancement layer OLED also exhibits outcoupling comparableto that of a typical OLED without the enhancement layer, see the plot ofFIG. 14B. The structure in FIG. 11 was fabricated and the emissionspectrum of the device is shown in FIG. 15. The cavity shows narrowedemission similar to the modeled result with a full-width half maximum of34-40 nm.

To summarize these observations, the rate constant of emission for anOLED emitter, when an enhancement layer is used, is strongly dependenton the emitter and device architecture. A light emitting materialpositioned 10 nm from the enhancement layer is expected to have itsexcited state decay rate constant increase is on the order of a factorof 5 and to be closer than either threshold distance. A typicalphosphorescent emitter has an emission rate constant between 1.25E7 to2E5 1/s, the expected emission rate in the presence of the enhancementlayer is 6.25E7 to 1E6 1/s. For emitters 5 nm from the enhancementlayer, the expected excited state decay rate constant increase is on theorder of a factor of 20. These rate constant enhancements aresignificantly greater than those possible with a microcavity or othertypically used methods of changing the photon density of states in anOLED where the rate enhancement is on the order of a factor of 1.5 to 2.

Optical modeling predicts that using an enhancement layer will increasethe average excited state decay rate constant by a factor of 1.5 to 400for the preferred embodiment. Theoretical estimates for more complexenhancement layers involving stacks of Ag and Al2O3 thin films place theincrease in the excited state decay rate constant as large as a factorof 1000. The exact relationship between improved emission rate constantand OLED durability is unknown. An increase in the excited state decayrate constant of the light emitter means that the emitter spends lesstime in the excited state and has less time to undergo an event whichdegrades the performance of the emitter. Without being constrained, theexpected increase in OLED durability using an enhancement layer is 1.5to 10000 times. Due to the potential for changes in emission lineshapeof the emitter when using the enhancement improvements to durability ofthe OLED may occur under either 1) constant current aging or 2) constantluminous efficacy aging. To be explicit, narrowing of the emissionlineshape or enhancement of off-peak emission by the enhancement layermay change the initial luminous efficacy (due to the sensitivity of thehuman eye) of the OLED. Thus, direct comparison of the aging rate underconstant luminous efficiency may show a different level of improvementthan aging the OLED under constant current.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. The present invention as claimed may therefore includevariations from the particular examples and preferred embodimentsdescribed herein, as will be apparent to one of skill in the art. It isunderstood that various theories as to why the invention works are notintended to be limiting.

1.-25. (canceled)
 26. A device comprising: a substrate; a firstelectrode; an organic emissive layer comprising an organic emissivematerial disposed over the electrode; an enhancement layer, comprising aplasmonic material exhibiting surface plasmon resonance thatnon-radiatively couples to the organic emissive material and transferexcited state energy from the emissive material to non-radiative mode ofsurface plasmon polaritons, disposed over the organic emissive layeropposite from the first electrode, wherein the enhancement layer isprovided no more than a threshold distance away from the organicemissive layer, wherein the organic emissive material has a totalnon-radiative decay rate constant and a total radiative decay rateconstant due to the presence of the enhancement layer, and the thresholddistance is where the total non-radiative decay rate constant is equalto the total radiative decay rate constant; and an outcoupling layerdisposed over the enhancement layer, wherein the outcoupling layerscatters the energy from the surface plasmon polaritons as photons tofree space.
 27. The device of claim 26, further comprising anintervening layer disposed between the enhancement layer and theoutcoupling layer, wherein the intervening layer has a thickness between1-10 nm and a refractive index from 0.1 to 4.0.
 28. The device of claim26, wherein the substrate is transparent and is disposed adjacent to theoutcoupling layer opposite from the enhancement layer, or the substrateis disposed adjacent to the first electrode opposite from the organicemissive layer.
 29. The device of claim 26, wherein the plasmonicmaterial is a metal selected from the group consisting of Ag, Au, Al,Pt, alloys of any combination of Ag, Au, Al, Pt, a doped oxide, and adoped nitride. 30.-31. (canceled)
 32. The device of claim 26, whereinthe enhancement layer is at least one set of gratings formed ofwavelength-sized or sub-wavelength-sized features that are arrangedperiodically, quasi-periodically, or randomly.
 33. The device of claim32, wherein the wavelength-sized features and the sub-wavelength-sizedfeatures have sharp edges.
 34. The device of claim 33, wherein thegrating has a periodic pattern, wherein the wavelength-sized orsub-wavelength-sized features are arranged uniformly along one directionwith a pitch of 100-2000 nm with a 10-90% duty cycle.
 35. (canceled) 36.The device of claim 33, wherein the enhancement layer is formed of twosets of gratings, wherein in each set of gratings, the wavelength-sizedor sub-wavelength-sized features are arranged non-uniformly along onedirection with a pitch of 100-2000 nm with a 10-90% duty cycle, whereineach set of gratings is oriented in different direction with a pitch of100-2000 nm with a 10-90% duty cycle in x-direction and y-direction. 37.(canceled)
 38. The device of claim 27, wherein the enhancement layer isa stack comprising a layer of plasmonic material and a layer ofdielectric material, wherein the plasmonic material layer is a metalselected from the group consisting of Ag, Au, Al, Pt, and alloys of anycombination of these materials.
 39. The device of claim 26, wherein theenhancement layer is a stack comprising a layer of plasmonic materialand a layer of an adhesion material layer, wherein the plasmonicmaterial is a metal selected from the group consisting of Ag, Au, Al,Pt, and alloys of any combination of these materials, and the adhesionmaterial is selected from the group consisting of Ni, Ti, Cr, Au, Ge,Si, and alloys of any combination of these materials.
 40. The device ofclaim 39, wherein the plasmonic material is Ag and the adhesion materialis Ge.
 41. The device of claim 39, wherein the layer of plasmonicmaterial has a thickness of 0.2 to 50 nm and the layer of adhesionmaterial has a thickness of 0.1 to 5 nm.
 42. The device of claim 26,wherein the outcoupling layer is comprised of an insulating material, asemiconducting material, a metal, or any combination thereof; theoutcoupling layer consists of two materials of differing refractiveindex along a plane parallel to the enhancement layer and the twomaterials have different refractive index, wherein the difference in thereal part of the refractive index is between 0.1 to 3.0; or theoutcoupling layer is at least one set of gratings formed ofwavelength-sized or sub-wavelength-sized features that are arrangedperiodically, quasi-periodically, or randomly. 43.-54. (canceled) 55.The device of claim 26, wherein the enhancement layer is a secondelectrode.
 56. The device of claim 26, wherein the device furthercomprises a second electrode disposed between the intervening layer andthe outcoupling layer. 57.-61. (canceled)